China's Sina Corp plans to ramp up investment in its hot Twitter-like product, Weibo, for the rest of the year in order to fend off competition from Tencent Holdings and attract a wider user base.

Sina, which has seen its stock soar on investor hopes that Weibo will be monetized sooner rather than later, reported a 39 percent decline in quarterly net profit after it invested heavily in the platform.

While investors want the site to be monetized quickly, Sina wants focus on expanding its current user base of white-collar users to include students and users in lower-tier cities. Sina hopes to have more than 200 million Weibo users by the end of the year, up from 140 million currently.

I hope we can build up scale quickly and move to monetization quickly, said Sina's chief executive, Charles Chao, on an earnings conference call.

I think the trend is to go deeper into the market in terms of geographical location so we can get more users, Chao said.

China's largest Internet firm, Tencent, has 160 million registered users for its Weibo product and is Sina's main competitor. Sina hopes to build scale to make it harder for its rivals to compete.

Sina said on Wednesday it earned a first-quarter net profit of $15 million, or 23 cents a share, compared with $24.4 million, or 37 cents a share, for the same period a year ago. Excluding one-off items, it earned 25 cents a share, missing the average forecast of earnings of 26 cents a share, according to Thomson Reuters I/B/E/S.

Sina said its Weibo investments will accelerate in the second and third quarter and will focus on product development and marketing.

I don't think anyone would be upset with them spending more money to develop this service of theirs because this is their growth opportunity, said Paul Wuh, a Hong Kong-based analyst at Samsung Securities.

Sina, which also operates China's top Internet portal, said first-quarter adjusted operating expenses ballooned 41 percent to $39.1 million, mainly due to higher personnel costs and increased marketing expenses for Weibo.

Chao said the firm plans to soon release new versions of Weibo and wants to move away from being an Internet portal to being a platform offering social media.

For the second quarter, Sina expects adjusted revenue of $112-$115 million, versus analysts' expectations of $115.3 million.

Mobile-related revenue fell 13 percent to about $21.3 million. Advertising revenue rose 33 percent to $72.3 million in the first quarter.

Revenue, excluding Sina's separate real estate advertising business, rose 19 percent to $95.5 million, beating analysts' expectations of $95.4 million.

Sina said in April it had no immediate plans to list Weibo and that it would diversify its operations into e-commerce and online games next year.

Sina's shares closed 0.2 percent lower at $119.80 on Nasdaq on Wednesday before the results were announced. They have risen about 74 percent this year on the monetization potential of Weibo, outperforming the 7 percent rise in the Nasdaq composite. Sina's stock was flat in after hours trade.

(Reporting by Melanie Lee; Editing by Matt Driskill)